There is also an i.MX BSP Porting Guide which covers the bootloader as part of the Linux BSP documentation, so I would recommend downloading the Linux BSP for the kernel you are interested in.

I’m not familiar with the Pytech boards in detail but you should be able to look for the closest board supported on the NXP BSP (most likely the i.MX6UL EVK) and use it as a starting point as some settings will be very similar.

You should be able to port to U-boot regardless of the boot mode or fuses, but you would need to describe the hardware of your board to U-boot.